1. Field of the Invention
The present invention generally relates to a secondary cell or battery and more particularly to a secondary cell utilizing a lithium ionic reaction or using zinc (or a zinc alloy) as a negative electrode, whereby the growth of a dendrite of lithium or zinc, caused by repetition of the charging and discharging, can be restrained and long battery life realized.
2. Description of the Related Art
Recently, it has been pointed out that there is a possibility of global warming owing to the green house effect of increasing atmospheric carbon dioxide. Thermal power plants convert thermal energy obtained by burning fossil fuels and so on into electrical energy. Burning of fossil fuels is, however, accompanied by carbon dioxide emissions. Thus, it has become difficult to increase the number of thermal power plants. Accordingly, it has been being proposed to perform what is called load leveling, namely, to a load an electric dynamo by storing night power, which is surplus power, in secondary batteries installed in ordinary homes for the purpose of utilizing the electric dynamo effectively. Moreover, there has been an ever-increasing demand for the development of light-weight high-energy-density secondary batteries for use in electric vehicles that do not discharge substances regarded as air pollution, which include COx, NOx and hydrocarbon, and of small-size light-weight high-performance secondary cells as power sources for portable equipment such as notebook personal computers, word processors, video cameras and portable telephones.
The development of one of such high-performance secondary cell, namely, a lithium-ion cell of the rocking chair type employing an intercalation compound, to which lithium ions are intercalated, as a positive active material (namely, an anode active material) and further employing carbon as a negative active material (namely, a cathode active material) has been advanced. A part of the lithium-ion cells of such a type is proceeding toward practical utilization. It cannot be said that presently available lithium-ion cells attain high energy density which is an essential feature of the lithium cell employing metallic lithium as a negative active material.
In the case of a lithium secondary cell, dendritic lithium is sometimes deposited on a negative electrode during charging operation. This phenomenon may result in causing internal short between positive and negative electrodes of the cell or in self-discharge.
Further, in the Journal of Applied Electrochemistry vol. 22 (1992), pp. 620 to 627, there has been described a lithium secondary cell that employs aluminum foil, the surface of which is etched, as a negative electrode.
This lithium secondary cell, however, does not have practically satisfactory performance because the growth of a dendrite occurs despite countermeasures taken by this secondary cell, when the cycle of charge and discharge is repeated a practical number of times.
Especially when the negative electrode is faced with the positive electrode in the proximity thereof across a separator, the life or duration of the cycle of charge and discharge sometimes decreases. Many of such lithium secondary cells actually produced have extremely short lives.
Similarly as in the case of the lithium secondary cells which employ lithium alloys or aluminum as negative electrodes, nickel-zinc batteries and air-zinc cells have drawbacks in that some of the produced batteries or cells of such types have extremely short lives.
Such being the case, the advent of high-energy-density long-cycle-life lithium secondary cells, nickel-zinc secondary cells, air-zinc batteries and bromine-zinc secondary batteries is awaited. The fact of the matter, however, is that the aforementioned problems have to be solved in conventional secondary cells and batteries of such types.
Accordingly, the present invention is directed to solving the aforementioned problems.
Further, it is an object of the present invention to provide high-energy-density long-cycle-life lithium secondary cells, nickel-zinc secondary cells, air-zinc batteries and bromine-zinc secondary batteries.
Moreover, it is another object of the present invention to provide a method for forming materials of the negative electrodes of such lithium secondary cells, nickel-zinc secondary cells, air-zinc batteries and bromine-zinc secondary batteries.
Furthermore, it is still another object of the present invention to provide a method for handling the materials of the negative electrodes of such lithium secondary cells, nickel-zinc secondary cells, air-zinc batteries and bromine-zinc secondary batteries.
To achieve the foregoing objects, in accordance with an aspect of the present invention, there is provided a secondary cell which includes positive and negative electrodes separated from each other by a separator in an electrolyte contained within a case therefor. Further, the negative electrode is made of metallic powder alloyed with at least a metal (hereunder sometimes referred to as an amphoteric metal) which reacts with both of an acid and an alkali. Moreover, the metallic powder of the present invention has a large specific surface area. Preferably, the specific surface area of this metallic powder is equal to or more than 10 m2/g and the grain size (or particle diameter) thereof is equal to or less than 100 xcexcm. The amphoteric metal is made of at least one kind of a metal selected from aluminum, zinc, tin and lead. Furthermore, it is preferable that this amphoteric metal is alloyed with at least one or more kind of metal from among nickel, cobalt, copper, titanium and iron. Moreover, what is called an element ratio expressed in percent of the weight of the latter metal to the amphoteric metal is preferably equal to or less than 60 percent.
Further, it is preferable that the metallic powder is micro-capsulated with a coating or film. The coating of this micro-capsule is preferably made of a material which neither reacts with the electrolyte and is nor soluble in an electrolyte solution and is thus stable and is permeable to ions involved in a cell reaction but is resistant to oxidizing in the presence of oxygen. Moreover, the coating is constituted by an insulator (or a semiconductor) having a molecular structure, in which clearances or pores larger than the ions involved in the cell reaction are provided. Preferably, a material of this coating is a metallic oxide. Furthermore, this metallic oxide is preferably the oxide of at least one kind of a metal from among tungsten, molybdenum, titanium, vanadium, niobium, zirconium, hafnium, tantalum and chromium. Preferably, another material of this coating is an organic polymer of a fluororesin, a silicone resin, a polyolefine of polyethylene or polypropylene, a titanium resin, a polymer of derivatives of a macrocyclic compound, or a polymer of derivatives of aromatic hydrocarbon. Additionally, this coating may be made of a composite material of a metallic oxide and an organic material.
Further, it is preferable that the surface of the negative electrode made of the micro-capsulated metallic powder is coated with an insulating film or a semiconductor film which is not soluble in the electrolyte solution and is permeable to the ions involved in the cell reaction but is not permeable to a lithium metal (or a zinc metal) serving as a negative active material. Moreover, it is desirable that the coating has the peak size of the clearances in the molecular structure or of the pores within a range of 0.15 to 100 nanometers. Furthermore, this coating may be made of an insulating film or a semiconductor film.
Additionally, secondary cells or batteries, to which the present invention can be applied, are lithium cells in which the oxidation and reduction of lithium ions are performed by charging and discharging reactions, and nickel-zinc secondary batteries, air-zinc secondary batteries and bromine-zinc secondary batteries, the negative electrode of which is made of at least a zinc metal.
Further, the secondary cell of the present invention is characterized in that the negative electrode is made of metallic powder alloyed with an amphoteric metal which reacts with both of an acid and an alkali, that the elution of the amphoteric metal is performed by selective etching thereof so that the specific surface area is increased. Furthermore, it is preferable that the amount of the amphoteric metal eluted by the etching is equal to or less than 60 percent. The metallic powder, whose specific surface area is increased, may be then etched by an acid so as to further increase the specific surface area. Moreover, the etched metallic powder used in the secondary cell of the present invention is kept or preserved by being cut off from air, or kept in an oxgen-free solvent.
In the alloying of the amphoteric metal powder, the amphoteric metal reacts with both of an acid and an alkali. Thus, metallic powder having pores and a high specific surface area can be obtained by selectively etching and removing the amphoteric metal from the alloy of the amphoteric metal powder.
The negative electrode of the secondary cell of the present invention is made of such porous metallic powder. Thereby, the specific surface area is increased. Further, the contact area between the negative electrode and the electrolyte solution is increased. Moreover, the diffusion of lithium ions (incidentally, in the case of an alkaline cell, hydrogen ions) into the negative electrode can be facilitated. Thus, the substantial current density of current flowing through the surface portion of the negative electrode can be reduced at the time of the charge and discharge. As a result, the growth of a dendrite of lithium or zinc is retarded.
Furthermore, the etched powder of the alloy of the amphoteric metal can be safely handled without greatly reducing the specific surface area thereof by micro-capsulating the powder by the coating or film made of an insulating material or a semiconductor material which neither reacts with the electrolyte and is not soluble in an electrolyte solution and is thus stable and is permeable to ions involved in a cell reaction.
Namely, metallic powder having a high specific surface area is very active in air and thus is easily oxidized. In some cases, such metallic powder ignites. In the case where the metallic powder having a high specific surface area is oxidized, the pores are crushed and the specific surface area is decreased. Thus, precautions are required to handle the metallic powder. For example, the metallic powder should be handled in an atmosphere from which oxygen has been removed. In the case of the secondary cell of the present invention, the micro-capsulation of the powder of the alloy of the amphoteric metal obviates the necessity of such handling of the powder.
Furthermore, this powder of the alloy of the amphoteric metal is used in a secondary cell having a lithium or zinc negative electrode. Thereby, the lithium or zinc negative active material, which is deposited on the surface of the powder of the alloy of the etched amphoteric metal at the time of charging, can be prevented from directly coming into contact with the electrolyte solution. Consequently, a secondary cell can be obtained, the performance of which never degrades.
Other features, objects and advantages of the present invention will become apparent from the following description of preferred embodiments with reference to the drawings in which like reference characters designate like or corresponding parts throughout several views.